Small (3.5-7.5mm). These flies are easily recognised by their small head situated almost below the hunchbacked thorax, the large ear-shaped lower calypter and large abdomen. Body colour dull or lustrous, usually black with white, yellow, or orange markings on thorax and abdomen. The small head largely occupied by the relatively large eyes, holoptic in both sexes. Wing clear or tinged; venation varying from complete to strongly reduced. Legs simple, femora sometimes swollen; empodium pulvilliform.

The larvae are endoparasitoids of spiders, usually one larva per spider, rarely two or more. The spider continues functioning normally until just prior to completion of the fourth instar. At the end of this stage the spider is largely eaten and the larva pupates outside the spider in the web the spider has woven at the very last. There are apparently no fixed host-parasitoid relationships at the species level. Adults may more or less abound locally but apart from species visiting flowers and feeding on nectar, they are rarely observed. They are mainly active on warm sunny days. Males may sometimes gather in large numbers at higher points in the landscape (hilltopping). Mating takes place in flight.